The invention is related to fuel cell systems that utilize gas recirculation for fuel cell stack temperature control, and in particular to molten carbonate fuel cell (MCFC) systems.
A fuel cell consists of two distinct reacting chambers commonly referred to as the anode and cathode. Fuel cell systems are constructed of multiple fuel cells operating in parallel and connected electrically in series. The multiple fuel cells are typically combined into one operating unit with the appropriate gas manifolding to form what is commonly referred to as a fuel cell stack.
FIG. 1 illustrates a schematic view of a conventional MCFC system 10. In the MCFC system 10, reformed fuel is delivered to an anode 14 of a fuel cell stack 16 through an anode feed inlet 12 where it reacts with carbonate ions to produce carbon dioxide, water, and electricity. Typically, 70% to 85% of the fuel heating value is consumed in the anode 14. Downstream from the anode 14, the remaining unreacted fuel is delivered to a combustor 18 through an anode exhaust outlet 20. The unreacted fuel is burned in the combustor 18 to produce carbon dioxide and water. Oxygen is supplied to the combustor 18 and may be either air and/or cathode exhaust gases. Various heat integration schemes have been proposed for recuperating both the heat in the cathode exhaust and the heat liberated in the combustor 18 for various purposes such as preheating, steam generation, and supplying the heat required for reforming fuel. The combustor exhaust is mixed with excess air and sent to a cathode 22 of the fuel cell stack 16 through a cathode feed inlet 24 where carbon dioxide and oxygen are reacted to regenerate the carbonate ions consumed at the anode 14.
The overall reaction in the fuel cell stack 16 is that of combustion. Heat is generated in the stack 16 through the heat of reaction of fuel as well as heat generated through ohmic losses. A typical fuel cell system utilizes recirculation of cooled cathode exhaust gases in order to control the fuel cell stack temperature. In some systems, anode gas recirculation is also used. The cathode exhaust gases are recirculated through a cathode exhaust outlet 26 and are directed through a recycle cooler 28. A recycle blower 30 is also used to recirculate the gas. The recycle blower 30 introduces various problems and limitations including decreased reliability, increased maintenance, increased auxiliary power consumption, increased plot space, and increased noise. In addition, recirculation of gases leads to larger gas flows and therefore larger diameter piping between the fuel cell stack 16 and the balance of plant.